With an engine loaded on a vehicle or the like, a fuel injection volume has so far been set in accordance with the amount of intake air so that an air-fuel ratio will become a preset target air-fuel ratio. Owing to, for example, changes in the operating state of the engine or variations in the characteristics of fuel injection valves, however, the desired volume of fuel may fail to be injected. Usually, therefore, feedback control over the fuel injection volume is exercised, as appropriate, based on exhaust air-fuel ratio information from an air-fuel ratio sensor (for example, a linear air-fuel ratio sensor (LAFS) or an O2 sensor) provided in an exhaust passage. With this feedback control, a feedback correction factor is set based on the exhaust air-fuel ratio information, and the fuel injection volume is corrected, as appropriate, in accordance with the feedback correction factor.
Moreover, the amount of deviation in the injection volume due to a specific variation of the fuel injection valve can also be corrected by feedback control. Separately, however, learning control for learning the deviation amount is performed to set a learning value, and correction of the deviation amount is made based on the learning value. It is preferred, for example, to perform the learning control at the time of replacing the fuel injection valve, and complete it in as short a time as possible. This is intended to suppress the deterioration of an exhaust gas due to the deviation of the injection volume.
In an engine having a first fuel injection valve (port injection valve) for injecting fuel into an intake passage (intake port) and a second fuel injection valve (cylinder injection valve) for injecting fuel into a combustion chamber, fuel injection volumes need to be corrected, as appropriate, for the first fuel injection valve and the second fuel injection valve, respectively. An example of the engine is designed to calculate the correction amount of each fuel injection valve in accordance with the injection sharing ratio between the port injection valve and the cylinder injection valve (see, for example, Patent Document 1).